The invention relates to high efficiency DC-to-DC switching power supplies and snubber circuits therefor.
It has long been known to provide a direct current to direct current (DC-to-DC) converter by switching current in a primary circuit to induce current at a higher voltage in a secondary circuit. Many circuits have been devised to sufficiently smooth the resulting secondary current to a level tolerable by the circuit load. Some of such circuits use a centre tapped transformer and two diodes which act to alternately direct current on each half waveform to charge a capacitor for bulk storage of energy to be supplied to the load. These circuits require an expensive transformer having a tapped winding, expensive switching devices and at least two diodes to achieve their goal. Such circuits may be found in U.S. Pats. 4,336,587 (Boettcher) and 4,276,588 (McLyman et Al).
Simpler secondary circuits have been devised and some illustrated in U.S. Pat. Nos. 4,365,171 (Archer); 4,355,353 (Farrer); 4,561,046 (Kuster) and 4,438,486 (Ferraro). The circuits shown in these patents all use the principle of half wave rectification and have the advantage that a simpler and cheaper transformer and a single diode are all that are required to charge the bulk storage capacitor. Unfortunately, these circuits have a disadvantage in that energy is transferred to the storage capacitor only on one half of the secondary voltage waveform. Thus, all of the energy of the first half of the waveform is lost.
Another problem encountered in virtually all DC-to-DC converters is the build up of extremely high voltages across a switching element in the primary circuit. As illustrated by the above patents, the switching element is used in the primary circuit to create increasing and decreasing current flow and hence increasing and decreasing magnetic flux in the primary winding. The primary winding, however, induces a voltage depending upon the rate of change of magnetic flux and therefore at the instant of switching the primary circuit open, the rate of change of flux is greatest and hence the voltage across the primary winding is greatest. This voltage simultaneously appears across the switching element which can lead to rapid degradation of the switching element.
Snubber circuits have been previously devised to reduce the voltage exerted across the switching element by providing a circuit path of reduced impedance. Such a circuit path allows the primary winding to be discharged while reducing the voltage across the switching element. Typically however, snubber circuits merely appear as a short circuit to the primary winding and thus energy due to the discharge of the primary winding manifests itself as heat in the snubber circuit.